Abstract

The TALE-class homeoprotein MEIS1 specifically collaborates with HOXA9 to drive myeloid leukemogenesis. Although MEIS1 alone has only a moderate effect on cell proliferation in vitro, it is essential for the development of HOXA9-induced leukemia in vivo. Here, using murine models of leukemogenesis, we have shown that MEIS1 promotes leukemic cell homing and engraftment in bone marrow and enhances cell-cell interactions and cytokine-mediated cell migration. We analyzed global DNA binding of MEIS1 in leukemic cells as well as gene expression alterations in MEIS1-deficent cells and identified synaptotagmin-like 1 (Sytl1, also known as Slp1) as the MEIS1 target gene that cooperates with Hoxa9 in leukemogenesis. Replacement of SYTL1 in MEIS1-deficent cells restored both cell migration and engraftment. Further analysis revealed that SYTL1 promotes cell migration via activation of the CXCL12/CXCR4 axis, as SYTL1 determines intracellular trafficking of CXCR4. Together, our results reveal that MEIS1, through induction of SYTL1, promotes leukemogenesis and supports leukemic cell homing and engraftment, facilitating interactions between leukemic cells and bone marrow stroma.

(A) Motility of 32Dcl3 cells with or without Sytl1 overexpression was observed during a 30-minute period of stimulation by CXCL12-coated beads. The original position of each cell is indicated as a white mark at 10, 20, or 30 minutes. See for videos of the same cells. Representative images of 3 independent experiments are shown. Scale bar: 20 μm. (B) CXCL12-induced chemotaxis was quantified by measuring the accumulated distance in 40 cells each for SYTL1-expressing and -nonexpressing 32Dcl3 cells. Average accumulated distances (in number of 280-nm units) are indicated in boxes (n = 3, *P < 0.05; **P < 0.01, Mann-Whitney test). (C) Frequent incorporation of CXCL12-coated beads in H9M1 and HΔM/SYTL1 cells. Cells were seeded into VCAM1-coated chambers. The cells were fixed with 4% paraformaldehyde 5 minutes after addition of CXCL12-coated beads. Representative images of 3 independent experiments (arrows) are shown. The number of cells incorporating CXCL12-coated beads was measured. Frequencies of positive cells are indicated as the mean ± SEM of 3 independent experiments (**P < 0.01, 2-tailed Student’s t test). Scale bar: 20 μm.

(A) Representative immunofluorescence of H9M1, HΔM, or HΔM/SYTL1 cells after the indicated incubation periods with CXCL12 (3 experiments). Alexa Fluor 350–conjugated wheat germ agglutinin (WGA) was used to indicate plasma membrane. Scale bar: 20 μm. (B) The expression of CXCR4 on the surfaces of H9M1, HΔM, or HΔM/SYTL1 cells was analyzed by flow cytometry at the indicated times after CXCL12 stimulation. The cells were incubated with the anti-CXCR4 antibody after (same staining condition as A) or before CXCL12 stimulation. The data are representative of 3 independent experiments. (C) ERK and AKT phosphorylation after CXCL12 stimulation in the indicated cell types; blots are representative of 3 independent experiments. (D) Juxtaposition of H9M1 or HΔM/SYTL1 cells and CAR cells in bone marrow. CAR cells are stained with anti-S100. Representative images of 3 independent experiments. Quantification with statistical analysis is shown in . Scale bar: 20 μm. (E) Proposed model showing enhanced membrane trafficking of CXCR4 after SYTL1 upregulation. CXCR4 is internalized and ubiquitinated upon binding to CXCL12. However, CXCR4 is rapidly provided from its reservoir on the limiting membrane of multivesicular bodies in the presence of SYTL1 and Rab27b.